Playing Multiple Rounds of defense on Eris will usually yield a sufficient amount of Plastids if all storage containers and enemies are looted.

Notes

Clan weapons hold one of the main reasons to farm Plastids, as components require bundles of 250 for each component. Since no planet lists Plastids first, and the fact that they drop in very low amounts, they are often considered the hardest resource to farm for, excluding rares.

Some planets that drop Plastids often drop the first resource much more often, and coupled with the low amount per drop, this can lead to very long missions with little Plastids gained.

Trivia

In real life, a plastid is a double-membrane organelle within plant cells, typically found in algae.